1. Technical Field of the Invention
The invention relates to an apparatus for optically scanning a document to generate digital image data corresponding to local optical densities of the document, such apparatus frequently being termed a scanner. The invention more particularly relates to a scanner provided with a fixed glass plate, one side of which defines an original image plane; a transporting mechanism for transporting the document in a sub-scanning direction in the original image plane; at least two linear sensors situated in a main scanning direction, each including an array of opto-electric converters, and each provided with a lens which images a linear observation area onto the sensor situated in the original plane, the observation areas of the sensors being substantially in line and partially overlapping at the ends.
2. Description of Related Art
In a scanner of the type mentioned above, a document to be scanned is moved in one direction by the transporting mechanism while the sensors cooperate to scan a stationary linear area that is oriented perpendicular to the transport direction of the document. In this way, the document may be scanned two-dimensionally. The direction of the area scanned by the scanners is generally termed the main scanning direction and the transport direction is the sub-scanning direction.
In order to ensure that the document is completely scanned during the scanning operation, the sensors and their lenses are usually disposed such that their observation areas overlap somewhat at the ends. In practice, it is impossible to mount the sensors such that their observation areas are exactly in contact. Furthermore, maintaining exact contact between the observation areas is also impossible because the scanner is typically subjected to vibrations and impacts particularly during transport of the scanner.
Within the overlapping observation areas, the two sensors deliver redundant digital image data. Thus, a choice has to be made as to which data are passed and which are not, a marginal condition being that the transition from one observation area to the other observation area should not be visible in the final image.
There are various conventional techniques to deal with the transition between two sensors. For example, U.S. Pat. No. 4,657,745 describes a scanner provided with a movable glass plate on which the document to be scanned is placed. A narrow black line is provided in the transport direction on the glass plate and outside the area for documents. When the glass plate is in the home position, the black line is situated within the overlapping portions of the observation areas of the two sensors. The adjustment procedure searches for the image of the black line in the image data generated by the two sensors. The position of this black line in the image data is used as the transition from one sensor to the other to generate digital image data during scanning. The black line does not necessarily have to lie in the same plane as the document, but can, for example, be lower, e.g. at the bottom of the glass plate. In that case, the result of the adjustment procedure is corrected for parallax, by shifting the transition point, for both sensors, a predetermined number of pixels from their end.
U.S. Pat. No. 4,870,505 also relates to a scanner with a movable glass plate to receive a document. In the '505 patent, a test pattern (correction reference chart) having line patterns oriented in the direction of transport and triangle patterns that are disposed on the movable glass plate outside the area for documents. This test pattern is scanned during an adjustment procedure. In this way, the relative positions and the skew positions are calculated from the points of intersection of the test pattern with the observation areas of the separate sensors. The connecting points of the sensors are calculated from these data. Also, a correction can be carried out for a connection error in the transport direction between two skewed, adjoining sensors by buffering the image data of some scan lines during scanning and sourcing the outgoing image data from different image lines.
The above patents consistently utilize a movable glass plate on which a separate test pattern is provided outside the area reserved for the document to be scanned. However, there are also scanners which do not have such a movable glass transport system but in which the document to be scanned is conveyed by pairs of rollers over a narrow stationary glass plate mounted in the main scanning direction. This construction is simpler while maintaining accuracy and is certainly preferable for large document formats.
The above-mentioned adjustment techniques can only be used with a scanner having a stationary glass plate by making the test patterns in the form of documents (e.g. test cards). In that case, it is difficult to ensure that the test pattern is disposed with sufficient accuracy at the correct position.
To solve this problem, U.S. Pat. No. 5,117,295 replaces the test pattern with an aligning element situated between the lenses and the original image plane at a short distance from the latter in a position such that its perpendicular projection onto the original image plane is situated within the overlapping ends of two adjoining observation areas. This aligning element is formed by a thin wire in the transport direction and is disposed beneath the glass plate.
The pixels which correspond to the aligning element are then used in the '295 patent to determine the transition from one sensor to the next in the digital image data of the two observation areas. This is explained by reference to FIGS. 1A and 1B.
FIG. 1A is a sketch showing the principle of a scanner with two CCD sensors S1 and S2, on each of which part of the glass plate 1 (for the sake of simplicity the glass thickness is not shown) is projected by a lens L1 and L2 respectively. The portion of the glass plate projected onto each sensor is also termed an observation area (O1 and O2 are the observation areas of sensors S1 and S2, respectively). The transport direction of the document D to be scanned on the top of the glass plate 1 is perpendicular to the drawing plane.
In the overlapping part of the observation areas O1 and O2 of the sensors S1 and S2, an aligning element 2 in the form of a wire perpendicular to the drawing plane is disposed beneath the glass plate 1. Both sensors S1 and S2 thus detect aligning element 2. The projection P of the aligning element 2 onto the plane of the document D for scanning (the original image plane) is selected as the transition point where the digital image data from sensor S1 and the digital image data from sensor S2 are fitted together. The image data from the ends of the sensors outside the position corresponding to point P are not used.
FIG. 1B shows the right-hand part of the image projected on to the sensor S1. The adjustment procedure searches for the middle 3 of the image of the aligning element 2 on the sensor and the measurement data of that point (pixel) is used as a reference. From point 3 a fixed number N of pixels are counted to find the transition point P (4). The number N can be determined experimentally or it can be preselected, whereafter the distance of the aligning element 2 from the glass plate 1 is adjusted until the required effect is obtained. During the scanning of documents, the measurement data from sensor elements between point 4 and end 5 of the projected image, and also the end of the sensor, are discarded. The remaining measured values are added to those from the other sensor, which has undergone the same procedure for determining the connecting point.
In this way, an accurate connection of the digital image from the various sensors can be obtained even for a scanner having a transport system without a moving glass plate. The advantage of these techniques is that any change of the mechanical adjustment of the sensors can be corrected automatically using digital techniques that do not require any mechanical action by a maintenance engineer.
However, the conventional art applies only to a distortion in the adjustment in the direction of the sensors themselves, the main scanning direction. A displacement of the sensors resulting in their observation areas no longer being in line, i.e. if the sensors are displaced relative to one another in the sub-scanning direction, cannot be determined in the above-described manner, let alone corrected. Such displacement, however, is quite possible and may occur, for example, if the apparatus is moved, transported or subjected to a physical shock. It is therefore desirable to be able to automatically compensate for displacement of the sensors in the sub-scanning direction as well as the main scanning direction.